Active composite and its use as reaction medium

ABSTRACT

An active composite consists of a compressed support and an agent which reacts with a gas. According to the invention, the compressed support comprises recompressed expanded graphite having a density between 0.02 and 1.5. A process for implementing gas-solid type reactions in which said active composite is used as the reaction medium is also described.

The present invention relates to an active composite comprising acompressed support and an agent which is reactive towards a gas. Thepresent invention also relates to a method for implementation ofgas/solid or gas/liquid physicochemical processes using such an activecomposite as reaction medium

In some sectors, such as, for example, that of chemical heat pumps basedon the thermicity of the reaction between a solid and a gas, or of theadsorption of a gas on a solid, a mixture of a finely divided material,such as expanded graphite, and a solid reactant, for example a salt oran adsorbent such as a zeolite, is used. The mixture of expandedgraphite and this solid, which is the seat of a chemical reaction or ofa physical adsorption, has numerous advantages during a chemicalreaction or a physical adsorption between the solid and a gas. Theexpanded graphite is in the form of platelets or platelet particleshaving a very large specific surface area and permits the diffusion ofthe gas even in a confined medium.

The substantial improvement observed in the kinetics of reversiblesolid/gas reactions, as resulting from the mixture of the active solidwith natural graphite expanded by a thermal route, in the givenproportions by mass and compacted in a fixed volume, results from anexcellent permeability of the fixed bed prepared in this way to thereactive gas and in a thermal conductivity accompanied by a good heatexchange coefficient at the walls. Conventionally, the preparation ofsuch a reagent leads to a homogeneous isotropic bed, the isotropicconductivity of which lies at values of 0.5 to 2 Wm⁻¹ K⁻¹ depending onthe conditions of preparation (proportion and compacting), and theexchange coefficient of which at the walls is between 50 and 150 Wm⁻²K⁻¹.

Despite these advantages, the use of such mixtures in the granular statehas drawbacks because of the difficulty in obtaining truly homogeneousmixtures, the difficulty in handling these and the large volume whichthey occupy. Moreover, the reactant, which is frequently hygroscopic,has a tendency to absorb moisture if the mixing operation is prolonged;a subsequent dehydration is lengthy and even costly and may influencethe quality of the final product even after dehydration.

The aim of the present invention is, therefore, an active compositewhich is in the form of a block which can be handled easily and which,while preserving excellent permeability in one direction, has heattransfer properties in a preferred direction.

To this end, the invention proposes an active composite comprising acompressed support and an agent which is reactive towards a gas,characterised in that the compressed support comprises recompressedexpanded graphite having a density of between 0.02 and 1.5. Unlessstated otherwise, all values of density are expressed by conventionalunits (g/cm³ or g cm⁻³).

According to a preferred embodiment, the active composite contains from1 to 95% by weight of recompressed expanded graphite and from 99 to 5%by weight of active agent.

The invention also provides a method for the implementation of solid/gasor solid/liquid [sic] physicochemical processes, characterised in thatit uses, as reaction medium, an active composite comprising a compresssupport and an agent which is reactive towards a gas, the compressedsupport comprising recompressed expanded graphite having a density ofbetween 0.02 and 1.5.

Such a block of active composite is intended to be used to carry outeither a reaction of the solid/gas type, or an adsorption between a gasand a solid, or the absorption of a gas in a liquid, for example asaturated or unsaturated solution of a solid, or a reaction between agas and a liquid catalysed by a solid, or the seat of acondensation/evaporation of a gas, or, finally, a gas conversionreaction catalysed by a solid. Thus, the present invention provides amethod for carrying out either reactions of the gas/solid type, orgas/solid adsorption, or absorption of a gas in a liquid or, finally,catalytic conversion of a gas, using, as reaction medium, a block ofactive composite according to the invention.

The active composite according to the invention must have a very highheat transfer anisotropy which will result from the ordered compressionof the graphite platelets, which are very good heat conductors, whilepreserving a high porosity enabling good diffusion of the gas to eachactive site which is arranged substantially uniformly in the composite.

The active composite according to the invention, the density of which isbetween 0.02 and 1.5, has a high thermal conductivity anisotropy, thatis to say the thermal conductivity C₁ in a first direction D₁ of theactive composite is distinctly higher than that obtained in anotherdirection D₂ of the composite which is perpendicular to the firstdirection The active composite according to the invention has ananisotropy coefficient, which is the ratio C₁ /C₂, of between 5 and 150and preferably between 10 and 100.

Moreover, the active composite according to the invention must have acertain porosity enabling gases to reach the active sites.

The total porosity of the composite corresponds to the percentage voidwithin the said composite, which is easily obtained from its measureddensity compared with the density of natural graphite:2.2.

The important characteristic of the composite is the open porosityrepresentative of the accessibility of liquids and gases to the activesites. This open porosity is expressed as a percentage of the totalporosity. It is determined by measuring the amount of water, kept underatmospheric pressure, absorbed by the block to which a pressure of 0.02bar is applied, and the impact which this absorption has on the densityof the composite.

The open porosity of the composite is generally between 32 and 99.1% ofits total porosity. Within this range it will depend to a large extenton the density chosen for the composite in the application envisaged.

Other characteristics and advantages of the present invention willbecome more clearly apparent on reading the following description, whichis given with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a longitudinal section of an apparatus intended to compress apulverulent material, and

FIG. 2 is a longitudinal section of a device intended to impregnate acompressed pulverulent material in order to produce an active compositeaccording to the invention.

As shown in FIG. 1, an apparatus intended to compress a pulverulentmaterial in order to form a porous support comprises a cylinder 10having a flat base 12. A piston 14, mounted so that it can slide in aleaktight manner in the cylinder 10, is arranged so as to be movable inthe direction of the arrow 16.

The cylinder is precharged with an amount of pulverulent material to becompressed. In the present example, the material to be compressed isexpanded graphite prepared by exfoliation and having an initial densityof between 0.001 and 0.02, the expansion temperature generally beingbetween 150° and 1200° C. The expanded graphite is compressed in thecylinder 10 by the piston 14 until a support or block of the desireddensity and cohesion is obtained. The expanded graphite block achievessuch a cohesion when its density is higher than 0.02. The expandedgraphite may advantageously be compressed so as to have a density ofbetween 0.02 and 1.5.

In the example illustrated, the block of compressed expanded graphite isa cylinder, the base of which may have any shape, not necessarilycircular; more generally, it is possible to obtain any desired shapesother than cylindrical, either by combining several compressiondirections or by machining the block which will have to have asufficient cohesion.

As shown in FIG. 2, a device intended, for impregnating the block ofcompressed expanded graphite comprises a leaktight chamber 18 formed ina cylinder 20 closed by a leaktight cover 22. The dimensions of thechamber 18 are slightly larger than those of the block 24 to enable itto accommodate, on the one hand, the block 24 and, on the other hand, anamount of liquid product 26 intended to impregnate the block 24. If theactive agent selected is a solid, this must then be introduced in asuitable chosen liquid, either by dissolving in a solvent or by veryfine grinding and suspending, which operation then enables the liquidimpregnating agent to be produced. The cover 22 has an opening 28intended for connection to a vacuum source (not shown). The fall inpressure inside the unit 24 causes the liquid product 26 to penetrateinto the interstices of the porous medium or into the interior of theaccessible pores. The pressure applied at the opening 28 is in the range0.01 to 0.1 bar and preferably 0.02 bar. For a block having a volume of3 dm³, the impregnation time is generally between a few seconds and 1minute.

When the solvent used as impregnating agent is water it is, for example,quite sufficient to create a vacuum using a water pump.

When the active agent is a solid it is often advantageous to evaporatethe intermediate liquid which has served to produce the impregnatingagent, taking all precautions which enable the active agent to be lefthomogenously dispersed in the porosities of the support. Thus, if thisliquid is water, the impregnated block 24 must be heated in an oven, thetemperature of which is controlled to rise from 60° to 100° C. over aperiod of between 5 and 24 hours, this enabling the water to beevaporated from the solution. The oven is then brought to a temperatureof at least 200° C. so as to remove the water of crystallisation, itbeing possible for the holding time of the block at this temperature tobe between 1 h and 10 h.

This conditioning method can be used with numerous different activeagents, such as those proposed by way of example in Table I. An activeagent is understood to be, for example, a reactive solid, an adsorbentsolid, an absorbent liquid or a solid acting as catalyst.

                  TABLE 1                                                         ______________________________________                                        Nature of the                                                                 active agent/                                                                 gas interaction                                                                          Active agents  Gas                                                 ______________________________________                                        Solid/gas  halides        water                                               reactions  pseudohalides  NH.sub.3 and derivatives                            (reversible)                                                                             carbonates     (amines)                                                       sulphates                                                                     nitrates                                                                      oxides         CO.sub.2                                                                      SO.sub.2                                                                      SO.sub.3                                                       metals         O2 [sic]                                                       metal alloys   H.sub.2                                                                       hydrocarbons                                                   metal hydrides H.sub.2                                             liquid/gas aqueous solutions                                                                            water                                               absorption halides                                                            (reversible)                                                                             pseudohalides                                                      and saturated                                                                            carbonates                                                         liquid/gas sulphates                                                          absorption nitrates                                                           (reversible)                                                                             solutions in liquid                                                                          NH.sub.3 and                                                   NH.sub.3       derivatives                                                    halides                                                                       pseudohalides                                                                 carbonates                                                                    sulphates                                                                     nitrates                                                           Solid/gas  zeolite        water                                               adsorption active charcoal                                                                              methanol and                                        (reversible)                                                                             silica gel     derivatives                                                    phosphorus                                                                    pentoxide                                                          heterogeneous                                                                            Ni + C.sub.6 H.sub.6   H.sub.2                                     catalysis                                                                     ______________________________________                                    

In the case of chlorides, Table II specifies the nature of impregnatingliquids dissolving or suspending the active agent.

                  TABLE II                                                        ______________________________________                                                  Impregnating liquid                                                 Active Agent                                                                              Dissolving     suspending                                         ______________________________________                                                    water or                                                          CaCl.sub.2  alcohol, acetone                                                  MnCl.sub.2  alcohol        ether, liquid NH.sub.3                             BaCl.sub.2  alcohol                                                           NiCl.sub.2  alcohol, NH.sub.4 OH                                                                         liquid NH.sub.3                                    CuCl.sub.2  acetone                                                           CoCl.sub.2  alcohol, acetone                                                              ether                                                             SrCl.sub.2  alcohol, acetone                                                                             liqud NH.sub.3                                     NaCl        glycerol                                                          FeCl.sub.2  alcohol, acetone                                                                             ether                                              NH.sub.4 Cl alcohol, liquid NH.sub.3                                                                     acetone, ether                                     CdCl.sub.2  alcohol        acetone, ether                                     ______________________________________                                    

EXAMPLES 1 to 4

The characteristics of active composites according to the invention willnow be studied:

Example 1

Active composite comprising 25% of graphite, 75% of CaCl₂. Aftercompression of the expanded graphite on its own, a block having a massper unit volume of 0.2 g cm⁻³ was obtained Impregnation with a CaCl₂solution was carried out under vacuum; the block was then dried at 240°C. in order to remove the impregnating agent. The radial conductivity(that is to say in the direction perpendicular to the compressiondirection) of the block of active composite is 7 Wm⁻¹ K⁻¹ whereas itsaxial conductivity (that is to say in the direction parallel to thecompression direction) is less than 1 Wm⁻¹ K⁻¹. The reactivity of thismaterial towards methylamine was compared with that of an isotropicmixture of CaCl₂ with expanded graphite prepared in the sameproportions, and for the same thermodynamic conditions. The averagethermal power for 90% of the reaction, exchanged between the system andthe heat transfer fluid, per unit volume of the active composite, is 440kW m⁻³ in the case of the impregnated compressed material, whereas it is180 kW m⁻³ in the case of the simple physical mixture. The open porosityis 55% of the total porosity of the composite.

Example 2

An active composite comprising 50% of graphite, 50% of CaCl₂ is preparedfrom a block having a mass per unit volume of 0.2 g cm⁻³.

The radial conductivity of the block of active composite is 18 Wm⁻¹ K⁻¹and the contact coefficient is 860 Wm⁻² K⁻¹.

Under the same conditions as above, the specific outputs are,respectively, 770 kW m⁻³ and 240 kW m⁻³. The open porosity is 50% of thetotal porosity of the composite.

Example 3

Active composite containing 40% of graphite and 60% of MnCl₂. Under thesame conditions as above, starting from a block having a mass per unitvolume of 0.26 g/cm³ reacting with ammonia, the specific outputs are,respectively, 640 kW m⁻³ and 230 kW m⁻³. The open porosity is 52% of thetotal porosity of the composite.

Example 4

Active composite comprising 72% of graphite and 28% of CaCl₂. The radialconductivity is 24 Wm K⁻¹. Under the same conditions as the precedingexamples, for an active composite prepared from a graphite block havinga mass per unit volume of 0.3 g cm⁻² [sic], the specific outputs are,respectively, 960 kW m⁻³ and 260 kW m⁻³. The open porosity is 48% of thetotal porosity of the composite.

As the examples show, the active composite according to the inventionhas a density of between 0.02 and 1.5 and has a significant anisotropy,the conductivity in a first direction D₁ being higher than that in asecond direction D₂ perpendicular to the first. The platelet particlesof expanded graphite, the planes of which which [sic] have a very goodthermal conductivity (<20 W.m⁻¹ K⁻¹), are initially in all spacialdirections and order themselves in a single direction under the actionof the compression. The active composite has an anisotropy coefficientdefined by the ratio of the conductivities C₁ /C₂ which is 5 to 150 andpreferably between 10 and 150.

The active composite according to the invention may comprise from 1 to95% by weight of recompressed expanded graphite and from 5 to 99% byweight of active agent. Its contact coefficient is between 500 and 5000Wm⁻² K⁻¹.

The recompressed expanded graphite has a density of between 0.02 and 1.5and the open porosity accessible to liquids or to gases is between 99.1%and 32% of the total porosity of the composite.

The active composite according to the invention may constitute thereaction medium for a method for carrying out chemical reactions of thegas/solid type. By way of example, the gas is methylamine and the solidCaCl₂. In this case, the solid/gas reaction is as follows: ##STR1##

Other reactions are possible between the salts of Table II either withmethylamine or with ammonia.

The active composite according to the invention may be used in anymethod for carrying out physicochemical processes using a gas and eithera reactive solid, or an absorbent solid, or a saturated or unsaturatedabsorbent liquid, or a solid acting as catalyst, or, finally, the seatof a gas condensation/evaporation.

We claim:
 1. In an improved active composite for use in a gas-solidreaction heat pump; said active composite comprising an active agentreactive toward said gas so that said agent and said gas are capable ofinteracting for the operation of said heat pump; said agent beingdispersed in a graphite support media; wherein the improvementcomprises:said graphite support is expanded graphite which has beencompressed to a density of 0.02 g/cm³ to 1.5 g/cm³ ; said activecomposite comprises from 1% to 95% by weight of said graphite and from99% to 5% by weight of said active agent; said active composite has athermal conductivity anisotrophy coefficient C₁ /C₂ between 5-150wherein C₁ represents the thermal conductivity in a first direction andC₂ represents the thermal conductivity in a second direction which isperpendicular to said first direction; and said active composite has anopen porosity which is 32% to 99.1% of the total porosity of thecomposite, said open porosity being a value determined by measuring theamount of water, kept under atmospheric pressure, absorbed by a block ofsaid composite to which a pressure of 0.02 bar is applied.